Dan was Shizune’s uncle, the younger brother of one of Shizune’s parents. After the death of his younger sister, he began advocating for medical ninja to be assigned to every ninja squad. Tsunade shared a similar belief, and the two soon began dating because of it. During their time together Dan shared with Tsunade his dream of becoming Hokage, wanting the title so that he could better protect the villagers of Konoha. Dan was later fatally wounded during the Second Great Shinobi World War, and, despite Tsunade’s attempts to save him, died of his injuries.

(Source: Narutopedia)